RFC 7940
Representing Label Generation Rulesets Using XML
Internet Engineering Task Force (IETF) K. Davies Request for Comments: 7940 ICANN Category: Standards Track A. Freytag ISSN: 2070-1721 ASMUS, Inc. August 2016 Representing Label Generation Rulesets Using XMLAbstract
This document describes a method of representing rules for validating identifier labels and alternate representations of those labels using Extensible Markup Language (XML). These policies, known as "Label Generation Rulesets" (LGRs), are used for the implementation of Internationalized Domain Names (IDNs), for example. The rulesets are used to implement and share that aspect of policy defining which labels and Unicode code points are permitted for registrations, which alternative code points are considered variants, and what actions may be performed on labels containing those variants. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7940.
Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.Table of Contents
1. Introduction ....................................................4 2. Design Goals ....................................................5 3. Normative Language ..............................................6 4. LGR Format ......................................................6 4.1. Namespace ..................................................7 4.2. Basic Structure ............................................7 4.3. Metadata ...................................................8 4.3.1. The "version" Element ...............................8 4.3.2. The "date" Element ..................................9 4.3.3. The "language" Element ..............................9 4.3.4. The "scope" Element ................................10 4.3.5. The "description" Element ..........................10 4.3.6. The "validity-start" and "validity-end" Elements ...11 4.3.7. The "unicode-version" Element ......................11 4.3.8. The "references" Element ...........................12 5. Code Points and Variants .......................................13 5.1. Sequences .................................................14 5.2. Conditional Contexts ......................................15 5.3. Variants ..................................................16 5.3.1. Basic Variants .....................................16 5.3.2. The "type" Attribute ...............................17 5.3.3. Null Variants ......................................18 5.3.4. Variants with Reflexive Mapping ....................19 5.3.5. Conditional Variants ...............................20 5.4. Annotations ...............................................22 5.4.1. The "ref" Attribute ................................22 5.4.2. The "comment" Attribute ............................23 5.5. Code Point Tagging ........................................23
6. Whole Label and Context Evaluation .............................23 6.1. Basic Concepts ............................................23 6.2. Character Classes .........................................25 6.2.1. Declaring and Invoking Named Classes ...............25 6.2.2. Tag-Based Classes ..................................26 6.2.3. Unicode Property-Based Classes .....................26 6.2.4. Explicitly Declared Classes ........................28 6.2.5. Combined Classes ...................................29 6.3. Whole Label and Context Rules .............................30 6.3.1. The "rule" Element .................................31 6.3.2. The Match Operators ................................32 6.3.3. The "count" Attribute ..............................33 6.3.4. The "name" and "by-ref" Attributes .................34 6.3.5. The "choice" Element ...............................34 6.3.6. Literal Code Point Sequences .......................35 6.3.7. The "any" Element ..................................35 6.3.8. The "start" and "end" Elements .....................35 6.3.9. Example Context Rule from IDNA Specification .......36 6.4. Parameterized Context or When Rules .......................37 6.4.1. The "anchor" Element ...............................37 6.4.2. The "look-behind" and "look-ahead" Elements ........38 6.4.3. Omitting the "anchor" Element ......................40 7. The "action" Element ...........................................40 7.1. The "match" and "not-match" Attributes ....................41 7.2. Actions with Variant Type Triggers ........................41 7.2.1. The "any-variant", "all-variants", and "only-variants" Attributes .........................41 7.2.2. Example from Tables in the Style of RFC 3743 .......44 7.3. Recommended Disposition Values ............................45 7.4. Precedence ................................................45 7.5. Implied Actions ...........................................45 7.6. Default Actions ...........................................46 8. Processing a Label against an LGR ..............................47 8.1. Determining Eligibility for a Label .......................47 8.1.1. Determining Eligibility Using Reflexive Variant Mappings ...................................47 8.2. Determining Variants for a Label ..........................48 8.3. Determining a Disposition for a Label or Variant Label ....49 8.4. Duplicate Variant Labels ..................................50 8.5. Checking Labels for Collision .............................50 9. Conversion to and from Other Formats ...........................51 10. Media Type ....................................................51 11. IANA Considerations ...........................................52 11.1. Media Type Registration ..................................52 11.2. URN Registration .........................................53 11.3. Disposition Registry .....................................53
12. Security Considerations .......................................54 12.1. LGRs Are Only a Partial Remedy for Problem Space .........54 12.2. Computational Expense of Complex Tables ..................54 13. References ....................................................55 13.1. Normative References .....................................55 13.2. Informative References ...................................56 Appendix A. Example Tables ........................................58 Appendix B. How to Translate Tables Based on RFC 3743 into the XML Format ............................................63 Appendix C. Indic Syllable Structure Example ......................68 C.1. Reducing Complexity .......................................70 Appendix D. RELAX NG Compact Schema ...............................71 Acknowledgements ..................................................82 Authors' Addresses ................................................821. Introduction
This document specifies a method of using Extensible Markup Language (XML) to describe Label Generation Rulesets (LGRs). LGRs are algorithms used to determine whether, and under what conditions, a given identifier label is permitted, based on the code points it contains and their context. These algorithms comprise a list of permissible code points, variant code point mappings, and a set of rules that act on the code points and mappings. LGRs form part of an administrator's policies. In deploying Internationalized Domain Names (IDNs), they have also been known as IDN tables or variant tables. There are other kinds of policies relating to labels that are not normally covered by LGRs and are therefore not necessarily representable by the XML format described here. These include, but are not limited to, policies around trademarks, or prohibition of fraudulent or objectionable words. Administrators of the zones for top-level domain registries have historically published their LGRs using ASCII text or HTML. The formatting of these documents has been loosely based on the format used for the Language Variant Table described in [RFC3743]. [RFC4290] also provides a "model table format" that describes a similar set of functionality. Common to these formats is that the algorithms used to evaluate the data therein are implicit or specified elsewhere. Through the first decade of IDN deployment, experience has shown that LGRs derived from these formats are difficult to consistently implement and compare, due to their differing formats. A universal
format, such as one using a structured XML format, will assist by improving machine readability, consistency, reusability, and maintainability of LGRs. When used to represent a simple list of permitted code points, the format is quite straightforward. At the cost of some complexity in the resulting file, it also allows for an implementation of more sophisticated handling of conditional variants that reflects the known requirements of current zone administrator policies. Another feature of this format is that it allows many of the algorithms to be made explicit and machine implementable. A remaining small set of implicit algorithms is described in this document to allow commonality in implementation. While the predominant usage of this specification is to represent IDN label policy, the format is not limited to IDN usage and may also be used for describing ASCII domain name label rulesets, or other types of identifier labels beyond those used for domain names.2. Design Goals
The following goals informed the design of this format: o The format needs to be implementable in a reasonably straightforward manner in software. o The format should be able to be automatically checked for formatting errors, so that common mistakes can be caught. o An LGR needs to be able to express the set of valid code points that are allowed for registration under a specific administrator's policies. o An LGR needs to be able to express computed alternatives to a given identifier based on mapping relationships between code points, whether one-to-one or many-to-many. These computed alternatives are commonly known as "variants". o Variant code points should be able to be tagged with explicit dispositions or categories that can be used to support registry policy (such as whether to allocate the computed variant or to merely block it from usage or registration). o Variants and code points must be able to be stipulated based on contextual information. For example, some variants may only be applicable when they follow a certain code point or when the code point is displayed in a specific presentation form.
o The data contained within an LGR must be able to be interpreted
unambiguously, so that independent implementations that utilize
the contents will arrive at the same results.
o To the largest extent possible, policy rules should be able to be
specified in the XML format without relying on hidden or built-in
algorithms in implementations.
o LGRs should be suitable for comparison and reuse, such that one
could easily compare the contents of two or more to see the
differences, to merge them, and so on.
o As many existing IDN tables as practicable should be able to be
migrated to the LGR format with all applicable interpretation
logic retained.
These requirements are partly derived from reviewing the existing
corpus of published IDN tables, plus the requirements of ICANN's work
to implement an LGR for the DNS root zone [LGR-PROCEDURE]. In
particular, Section B of that document identifies five specific
requirements for an LGR methodology.
The syntax and rules in [RFC5892] and [RFC3743] were also reviewed.
It is explicitly not the goal of this format to stipulate what code
points should be listed in an LGR by a zone administrator. Which
registration policies are used for a particular zone are outside the
scope of this memo.
3. Normative Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
4. LGR Format
An LGR is expressed as a well-formed XML document [XML] that conforms
to the schema defined in Appendix D.
As XML is case sensitive, an LGR must be authored with the correct
casing. For example, the XML element names MUST be in lowercase as
described in this specification, and matching of attribute values is
only performed in a case-sensitive manner.
A document that is not well-formed, is non-conforming, or violates
other constraints specified in this specification MUST be rejected.
4.1. Namespace
The XML Namespace URI is "urn:ietf:params:xml:ns:lgr-1.0". See Section 11.2 for more information.4.2. Basic Structure
The basic XML framework of the document is as follows: <?xml version="1.0"?> <lgr xmlns="urn:ietf:params:xml:ns:lgr-1.0"> ... </lgr> The "lgr" element contains up to three sub-elements or sections. First is an optional "meta" element that contains all metadata associated with the LGR, such as its authorship, what it is used for, implementation notes, and references. This is followed by a required "data" element that contains the substantive code point data. Finally, an optional "rules" element contains information on rules for evaluating labels, if any, along with "action" elements providing for the disposition of labels and computed variant labels. <?xml version="1.0"?> <lgr xmlns="urn:ietf:params:xml:ns:lgr-1.0"> <meta> ... </meta> <data> ... </data> <rules> ... </rules> </lgr> A document MUST contain exactly one "lgr" element. Each "lgr" element MUST contain zero or one "meta" element, exactly one "data" element, and zero or one "rules" element; and these three elements MUST be in that order. Some elements that are direct or nested child elements of the "rules" element MUST be placed in a specific relative order to other elements for the LGR to be valid. An LGR that violates these constraints MUST be rejected. In other cases, changing the ordering would result in a valid, but different, specification.
In the following descriptions, required, non-repeating elements or attributes are generally not called out explicitly, in contrast to "OPTIONAL" ones, or those that "MAY" be repeated. For attributes that take lists as values, the elements MUST be space-separated.4.3. Metadata
The "meta" element expresses metadata associated with the LGR, and the element SHOULD be included so that the associated metadata are available as part of the LGR and cannot become disassociated. The following subsections describe elements that may appear within the "meta" element. The "meta" element can be used to identify the author or relevant contact person, explain the intended usage of the LGR, and provide implementation notes as well as references. Detailed metadata allow the LGR document to become self-documenting -- for example, if rendered in a human-readable format by an appropriate tool. Providing metadata pertaining to the date and version of the LGR is particularly encouraged to make it easier for interoperating consumers to ensure that they are using the correct LGR. With the exception of the "unicode-version" element, the data contained within is not required by software consuming the LGR in order to calculate valid labels or to calculate variants. If present, the "unicode-version" element MUST be used by a consumer of the table to identify that it has the correct Unicode property data to perform operations on the table. This ensures that possible differences in code point properties between editions of the Unicode Standard do not impact the product of calculations utilizing an LGR.4.3.1. The "version" Element
The "version" element is OPTIONAL. It is used to uniquely identify each version of the LGR. No specific format is required, but it is RECOMMENDED that it be the decimal representation of a single positive integer, which is incremented with each revision of the file. An example of a typical first edition of a document: <version>1</version> The "version" element may have an OPTIONAL "comment" attribute. <version comment="draft">1</version>
4.3.2. The "date" Element
The OPTIONAL "date" element is used to identify the date the LGR was posted. The contents of this element MUST be a valid ISO 8601 "full-date" string as described in [RFC3339]. Example of a date: <date>2009-11-01</date>4.3.3. The "language" Element
Each OPTIONAL "language" element identifies a language or script for which the LGR is intended. The value of the "language" element MUST be a valid language tag as described in [RFC5646]. The tag may refer to a script plus undefined language if the LGR is not intended for a specific language. Example of an LGR for the English language: <language>en</language> If the LGR applies to a script rather than a specific language, the "und" language tag SHOULD be used followed by the relevant script subtag from [RFC5646]. For example, for a Cyrillic script LGR: <language>und-Cyrl</language> If the LGR covers a set of multiple languages or scripts, the "language" element MAY be repeated. However, for cases of a script-specific LGR exhibiting insignificant admixture of code points from other scripts, it is RECOMMENDED to use a single "language" element identifying the predominant script. In the exceptional case of a multi-script LGR where no script is predominant, use Zyyy (Common): <language>und-Zyyy</language>
4.3.4. The "scope" Element
This OPTIONAL element refers to a scope, such as a domain, to which this policy is applied. The "type" attribute specifies the type of scope being defined. A type of "domain" means that the scope is a domain that represents the apex of the DNS zone to which the LGR is applied. For that type, the content of the "scope" element MUST be a domain name written relative to the root zone, in presentation format with no trailing dot. However, in the unique case of the DNS root zone, it is represented as ".". <scope type="domain">example.com</scope> There may be multiple "scope" tags used -- for example, to reflect a list of domains to which the LGR is applied. No other values of the "type" attribute are defined by this specification; however, this specification can be used for applications other than domain names. Implementers of LGRs for applications other than domain names SHOULD define the scope extension grammar in an IETF specification or use XML namespaces to distinguish their scoping mechanism distinctly from the base LGR namespace. An explanation of any custom usage of the scope in the "description" element is RECOMMENDED. <scope xmlns="http://example.com/ns/scope/1.0"> ... content per alternate namespace ... </scope>4.3.5. The "description" Element
The "description" element is an OPTIONAL, free-form element that contains any additional relevant description that is useful for the user in its interpretation. Typically, this field contains authorship information, as well as additional context on how the LGR was formulated and how it applies, such as citations and references that apply to the LGR as a whole. This field should not be relied upon for providing instructions on how to parse or utilize the data contained elsewhere in the specification. Authors of tables should expect that software applications that parse and use LGRs will not use the "description" element to condition the application of the LGR's data and rules.
The element has an OPTIONAL "type" attribute, which refers to the Internet media type [RFC2045] of the enclosed data. Typical types would be "text/plain" or "text/html". The attribute SHOULD be a valid media type. If supplied, it will be assumed that the contents are of that media type. If the description lacks a "type" value, it will be assumed to be plain text ("text/plain").4.3.6. The "validity-start" and "validity-end" Elements
The "validity-start" and "validity-end" elements are OPTIONAL elements that describe the time period from which the contents of the LGR become valid (are used in registry policy) and time when the contents of the LGR cease to be used, respectively. The dates MUST conform to the "full-date" format described in Section 5.6 of [RFC3339]. <validity-start>2014-03-12</validity-start>4.3.7. The "unicode-version" Element
Whenever an LGR depends on character properties from a given version of the Unicode Standard, the version number used in creating the LGR MUST be listed in the form x.y.z, where x, y, and z are positive decimal integers (see [Unicode-Versions]). If any software processing the table does not have access to character property data of the requisite version, it MUST NOT perform any operations relating to whole-label evaluation relying on Unicode character properties (Section 6.2.3). The value of a given Unicode character property may change between versions of the Unicode Character Database [UAX44], unless such change has been explicitly disallowed in [Unicode-Stability]. It is RECOMMENDED to only reference properties defined as stable or immutable. As an alternative to referencing the property, the information can be presented explicitly in the LGR. <unicode-version>6.3.0</unicode-version> It is not necessary to include a "unicode-version" element for LGRs that do not make use of Unicode character properties; however, it is RECOMMENDED.
4.3.8. The "references" Element
An LGR may define a list of references that are used to associate various individual elements in the LGR to one or more normative references. A common use for references is to annotate that code points belong to an externally defined collection or standard or to give normative references for rules. References are specified in an OPTIONAL "references" element containing one or more "reference" elements, each with a unique "id" attribute. It is RECOMMENDED that the "id" attribute be a zero-based integer; however, in addition to digits 0-9, it MAY contain uppercase letters A-Z, as well as a period, hyphen, colon, or underscore. The value of each "reference" element SHOULD be the citation of a standard, dictionary, or other specification in any suitable format. In addition to an "id" attribute, a "reference" element MAY have a "comment" attribute for an optional free-form annotation. <references> <reference id="0">The Unicode Consortium. The Unicode Standard, Version 8.0.0, (Mountain View, CA: The Unicode Consortium, 2015. ISBN 978-1-936213-10-8) http://www.unicode.org/versions/Unicode8.0.0/</reference> <reference id="1">Big-5: Computer Chinese Glyph and Character Code Mapping Table, Technical Report C-26, 1984</reference> <reference id="2" comment="synchronized with Unicode 6.1"> ISO/IEC 10646:2012 3rd edition</reference> ... </references> ... <data> <char cp="0620" ref="0 2" /> ... </data> A reference is associated with an element by using its id as part of an optional "ref" attribute (see Section 5.4.1). The "ref" attribute may be used with many kinds of elements in the "data" or "rules" sections of the LGR, most notably those defining code points, variants, and rules. However, a "ref" attribute may not occur in certain kinds of elements, including references to named character classes or rules. See below for the description of these elements.
5. Code Points and Variants
The bulk of an LGR is a description of which set of code points is eligible for a given label. For rulesets that perform operations that result in potential variants, the code point-level relationships between variants need to also be described. The code point data is collected within the "data" element. Within this element, a series of "char" and "range" elements describe eligible code points or ranges of code points, respectively. Collectively, these are known as the repertoire. Discrete permissible code points or code point sequences (see Section 5.1) are declared with a "char" element. Here is a minimal example declaration for a single code point, with the code point value given in the "cp" attribute: <char cp="002D"/> As described below, a full declaration for a "char" element, whether or not it is used for a single code point or for a sequence (see Section 5.1), may have optional child elements defining variants. Both the "char" and "range" elements can take a number of optional attributes for conditional inclusion, commenting, cross-referencing, and character tagging, as described below. Ranges of permissible code points may be declared with a "range" element, as in this minimal example: <range first-cp="0030" last-cp="0039"/> The range is inclusive of the first and last code points. Any additional attributes defined for a "range" element act as if applied to each code point within. A "range" element has no child elements. It is always possible to substitute a list of individually specified code points for a "range" element. The reverse is not necessarily the case. Whenever such a substitution is possible, it makes no difference in processing the data. Tools reading or writing the LGR format are free to aggregate sequences of consecutive code points of the same properties into "range" elements. Code points MUST be represented according to the standard Unicode convention but without the prefix "U+": they are expressed in uppercase hexadecimal and are zero-padded to a minimum of 4 digits.
The rationale for not allowing other encoding formats, including native Unicode encoding in XML, is explored in [UAX42]. The XML conventions used in this format, such as element and attribute names, mirror this document where practical and reasonable to do so. It is RECOMMENDED to list all "char" elements in ascending order of the "cp" attribute. Not doing so makes it unnecessarily difficult for authors and reviewers to check for errors, such as duplications, or to review and compare against listing of code points in other documents and specifications. All "char" elements in the "data" section MUST have distinct "cp" attributes. The "range" elements MUST NOT specify code point ranges that overlap either another range or any single code point "char" elements. An LGR that defines the same code point more than once by any combination of "char" or "range" elements MUST be rejected.5.1. Sequences
A sequence of two or more code points may be specified in an LGR -- for example, when defining the source for n:m variant mappings. Another use of sequences would be in cases when the exact sequence of code points is required to occur in order for the constituent elements to be eligible, such as when some code point is only eligible when preceded or followed by a certain code point. The following would define the eligibility of the MIDDLE DOT (U+00B7) only when both preceded and followed by the LATIN SMALL LETTER L (U+006C): <char cp="006C 00B7 006C" comment="Catalan middle dot"/> All sequences defined this way must be distinct, but sub-sequences may be defined. Thus, the sequence defined here may coexist with single code point definitions such as: <char cp="006C" /> As an alternative to using sequences to define a required context, a "char" or "range" element may specify a conditional context using an optional "when" attribute as described below in Section 5.2. Using a conditional context is more flexible because a context is not limited to a specific sequence of code points. In addition, using a context allows the choice of specifying either a prohibited or a required context.
5.2. Conditional Contexts
A conditional context is specified by a rule that must be satisfied (or, alternatively, must not be satisfied) for a code point in a given label, often at a particular location in a label. To specify a conditional context, either a "when" or "not-when" attribute may be used. The value of each "when" or "not-when" attribute is a context rule as described below in Section 6.3. This rule can be a rule evaluating the whole label or a parameterized context rule. The context condition is met when the rule specified in the "when" attribute is matched or when the rule in the "not-when" attribute fails to match. It is an error to reference a rule that is not actually defined in the "rules" element. A parameterized context rule (see Section 6.4) defines the context immediately surrounding a given code point; unlike a sequence, the context is not limited to a specific fixed code point but, for example, may designate any member of a certain character class or a code point that has a certain Unicode character property. Given a suitable definition of a parameterized context rule named "follows-virama", this example specifies that a ZERO WIDTH JOINER (U+200D) is restricted to immediately follow any of several code points classified as virama: <char cp="200D" when="follows-virama" /> For a complete example, see Appendix A. In contrast, a whole label rule (see Section 6.3) specifies a condition to be met by the entire label -- for example, that it must contain at least one code point from a given script anywhere in the label. In the following example, no digit from either range may occur in a label that mixes digits from both ranges: <data> <range first-cp="0660" last-cp="0669" not-when="mixed-digits" tag="arabic-indic-digits" /> <range first-cp="06F0" last-cp="06F9" not-when="mixed-digits" tag="extended-arabic-indic-digits" /> </data> (See Section 6.3.9 for an example of the "mixed-digits" rule.)
The OPTIONAL "when" or "not-when" attributes are mutually exclusive. They MAY be applied to both "char" and "range" elements in the "data" element, including "char" elements defining sequences of code points, as well as to "var" elements (see Section 5.3.5). If a label contains one or more code points that fail to satisfy a conditional context, the label is invalid (see Section 7.5). For variants, the conditional context restricts the definition of the variant to the case where the condition is met. Outside the specified context, a variant is not defined.5.3. Variants
Most LGRs typically only determine simple code point eligibility, and for them, the elements described so far would be the only ones required for their "data" section. Others additionally specify a mapping of code points to other code points, known as "variants". What constitutes a variant code point is a matter of policy and varies for each implementation. The following examples are intended to demonstrate the syntax; they are not necessarily typical.5.3.1. Basic Variants
Variant code points are specified using one of more "var" elements as children of a "char" element. The target mapping is specified using the "cp" attribute. Other, optional attributes for the "var" element are described below. For example, to map LATIN SMALL LETTER V (U+0076) as a variant of LATIN SMALL LETTER U (U+0075): <char cp="0075"> <var cp="0076"/> </char> A sequence of multiple code points can be specified as a variant of a single code point. For example, the sequence of LATIN SMALL LETTER O (U+006F) then LATIN SMALL LETTER E (U+0065) might hypothetically be specified as a variant for a LATIN SMALL LETTER O WITH DIAERESIS (U+00F6) as follows: <char cp="00F6"> <var cp="006F 0065"/> </char> The source and target of a variant mapping may both be sequences but not ranges.
If the source of one mapping is a prefix sequence of the source for another, both variant mappings will be considered at the same location in the input label when generating permuted variant labels. If poorly designed, an LGR containing such an instance of a prefix relation could generate multiple instances of the same variant label for the same original label, but with potentially different dispositions. Any duplicate variant labels encountered MUST be treated as an error (see Section 8.4). The "var" element specifies variant mappings in only one direction, even though the variant relation is usually considered symmetric; that is, if A is a variant of B, then B should also be a variant of A. The format requires that the inverse of the variant be given explicitly to fully specify symmetric variant relations in the LGR. This has the beneficial side effect of making the symmetry explicit: <char cp="006F 0065"> <var cp="00F6"/> </char> Variant relations are normally not only symmetric but also transitive. If A is a variant of B and B is a variant of C, then A is also a variant of C. As with symmetry, these transitive relations are only part of the LGR if spelled out explicitly. Implementations that require an LGR to be symmetric and transitive should verify this mechanically. All variant mappings are unique. For a given "char" element, all "var" elements MUST have a unique combination of "cp", "when", and "not-when" attributes. It is RECOMMENDED to list the "var" elements in ascending order of their target code point sequence. (For "when" and "not-when" attributes, see Section 5.3.5.)5.3.2. The "type" Attribute
Variants may be tagged with an OPTIONAL "type" attribute. The value of the "type" attribute may be any non-empty value not starting with an underscore and not containing spaces. This value is used to resolve the disposition of any variant labels created using a given variant. (See Section 7.2.) By default, the values of the "type" attribute directly describe the target policy status (disposition) for a variant label that was generated using a particular variant, with any variant label being assigned a disposition corresponding to the most restrictive variant type. Several conventional disposition values are predefined below in Section 7. Whenever these values can represent the desired policy, they SHOULD be used.
<char cp="767C">
<var cp="53D1" type="allocatable"/>
<var cp="5F42" type="blocked"/>
<var cp="9AEA" type="blocked"/>
<var cp="9AEE" type="blocked"/>
</char>
By default, if a variant label contains any instance of one of the
variants of type "blocked", the label would be blocked, but if it
contained only instances of variants to be allocated, it could be
allocated. See the discussion about implied actions in Section 7.6.
The XML format for the LGR makes the relation between the values of
the "type" attribute on variants and the resulting disposition of
variant labels fully explicit. See the discussion in Section 7.2.
Making this relation explicit allows a generalization of the "type"
attribute from directly reflecting dispositions to a more
differentiated intermediate value that is then used in the resolution
of label disposition. Instead of the default action of applying the
most restrictive disposition to the entire label, such a generalized
resolution can be used to achieve additional goals, such as limiting
the set of allocatable variant labels or implementing other policies
found in existing LGRs (see, for example, Appendix B).
Because variant mappings MUST be unique, it is not possible to define
the same variant for the same "char" element with different "type"
attributes (however, see Section 5.3.5).
5.3.3. Null Variants
A null variant is a variant string that maps to no code point. This
is used when a particular code point sequence is considered
discretionary in the context of a whole label. To specify a null
variant, use an empty "cp" attribute. For example, to mark a string
with a ZERO WIDTH NON-JOINER (U+200C) to the same string without the
ZERO WIDTH NON-JOINER:
<char cp="200C">
<var cp=""/>
</char>
This is useful in expressing the intent that some code points in a
label are to be mapped away when generating a canonical variant of
the label. However, in tables that are designed to have symmetric
variant mappings, this could lead to combinatorial explosion if not
handled carefully.
The symmetric form of a null variant is expressed as follows:
<char cp="">
<var cp="200C" type="invalid" />
</char>
A "char" element with an empty "cp" attribute MUST specify at least
one variant mapping. It is strongly RECOMMENDED to use a type of
"invalid" or equivalent when defining variant mappings from null
sequences, so that variant mappings from null sequences are removed
in variant label generation (see Section 5.3.2).
5.3.4. Variants with Reflexive Mapping
At first glance, there seems to be no call for adding variant
mappings for which source and target code points are the same -- that
is, for which the mapping is reflexive, or, in other words, an
identity mapping. Yet, such reflexive mappings occur frequently in
LGRs that follow [RFC3743].
Adding a "var" element allows both a type and a reference id to be
specified for it. While the reference id is not used in processing,
the type of the variant can be used to trigger actions. In permuting
the label to generate all possible variants, the type associated with
a reflexive variant mapping is applied to any of the permuted labels
containing the original code point.
In the following example, let's assume that the goal is to allocate
only those labels that contain a variant that is considered
"preferred" in some way. As defined in the example, the code point
U+3473 exists both as a variant of U+3447 and as a variant of itself
(reflexive mapping). Assuming an original label of "U+3473 U+3447",
the permuted variant "U+3473 U+3473" would consist of the reflexive
variant of U+3473 followed by a variant of U+3447. Given the variant
mappings as defined here, the types for both of the variant mappings
used to generate that particular permutation would have the value
"preferred":
<char cp="3447" ref="0">
<var cp="3473" type="preferred" ref="1 3" />
</char>
<char cp="3473" ref="0">
<var cp="3447" type="blocked" ref="1 3" />
<var cp="3473" type="preferred" ref="0" />
</char>
Having established the variant types in this way, a set of actions could be defined that return a disposition of "allocatable" or "activated" for a label consisting exclusively of variants with type "preferred", for example. (For details on how to define actions based on variant types, see Section 7.2.1.) In general, using reflexive variant mappings in this manner makes it possible to calculate disposition values using a uniform approach for all labels, whether they consist of mapped variant code points, original code points, or a mixture of both. In particular, the dispositions for two otherwise identical labels may differ based on which variant mappings were executed in order to generate each of them. (For details on how to generate variants and evaluate dispositions, see Section 8.) Another useful convention that uses reflexive variants is described below in Section 7.2.1.5.3.5. Conditional Variants
Fundamentally, variants are mappings between two sequences of code points. However, in some instances, for a variant relationship to exist, some context external to the code point sequence must also be considered. For example, a positional context may determine whether two code point sequences are variants of each other. An example of that are Arabic code points, which can have different forms based on position, with some code points sharing forms, thus making them variants in the positions corresponding to those forms. Such positional context cannot be solely derived from the code point by itself, as the code point would be the same for the various forms. As described in Section 5.2, an OPTIONAL "when" or "not-when" attribute may be given for any "var" element to specify required or prohibited contextual conditions under which the variant is defined. Assuming that the "rules" element contains suitably defined rules for "arabic-isolated" and "arabic-final", the following example shows how to mark ARABIC LETTER ALEF WITH WAVY HAMZA BELOW (U+0673) as a variant of ARABIC LETTER ALEF WITH HAMZA BELOW (U+0625), but only when it appears in its isolated or final forms: <char cp="0625"> <var cp="0673" when="arabic-isolated"/> <var cp="0673" when="arabic-final"/> </char>
While a "var" element MUST NOT contain multiple conditions (it is only allowed a single "when" or "not-when" attribute), multiple "var" elements using the same mapping MAY be specified with different "when" or "not-when" attributes. The combination of mapping and conditional context defines a unique variant. For each variant label, care must be taken to ensure that at most one of the contextual conditions is met for variants with the same mapping; otherwise, duplicate variant labels would be created for the same input label. Any such duplicate variant labels MUST be treated as an error; see Section 8.4. Two contexts may be complementary, as in the following example, which shows ARABIC LETTER TEH MARBUTA (U+0629) as a variant of ARABIC LETTER HEH (U+0647), but with two different types. <char cp="0647" > <var cp="0629" not-when="arabic-final" type="blocked" /> <var cp="0629" when="arabic-final" type="allocatable" /> </char> The intent is that a label that uses U+0629 instead of U+0647 in a final position should be considered essentially the same label and, therefore, allocatable to the same entity, while the same substitution in a non-final position leads to labels that are different, but considered confusable, so that either one, but not both, should be delegatable. For symmetry, the reverse mappings must exist and must agree in their "when" or "not-when" attributes. However, symmetry does not apply to the other attributes. For example, these are potential reverse mappings for the above: <char cp="0629" > <var cp="0647" not-when="arabic-final" type="allocatable" /> <var cp="0647" when="arabic-final" type="allocatable" /> </char> Here, both variants have the same "type" attribute. While it is tempting to recognize that, in this instance, the "when" and "not-when" attributes are complementary; therefore, between them they cover every single possible context, it is strongly RECOMMENDED to use the format shown in the example that makes the symmetry easily verifiable by parsers and tools. (The same applies to entries created for transitivity.)
Arabic is an example of a script for which such conditional variants have been implemented based on the joining contexts for Arabic code points. The mechanism defined here supports other forms of conditional variants that may be required by other scripts.5.4. Annotations
Two attributes, the "ref" and "comment" attributes, can be used to annotate individual elements in the LGR. They are ignored in machine-processing of the LGR. The "ref" attribute is intended for formal annotations and the "comment" attribute for free-form annotations. The latter can be applied more widely.5.4.1. The "ref" Attribute
Reference information MAY optionally be specified by a "ref" attribute consisting of a space-delimited sequence of reference identifiers (see Section 4.3.8). <char cp="5220" ref="0"> <var cp="5220" ref="5"/> <var cp="522A" ref="2 3"/> </char> This facility is typically used to give source information for code points or variant relations. This information is ignored when machine-processing an LGR. If applied to a range, the "ref" attribute applies to every code point in the range. All reference identifiers MUST be from the set declared in the "references" element (see Section 4.3.8). It is an error to repeat a reference identifier in the same "ref" attribute. It is RECOMMENDED that identifiers be listed in ascending order. In addition to "char", "range", and "var" elements in the "data" section, a "ref" attribute may be present for a number of element types contained in the "rules" element as described below: actions and literals ("char" inside a rule), as well as for definitions of rules and classes, but not for references to named character classes or rules using the "by-ref" attribute defined below. (The use of the "by-ref" and "ref" attributes is mutually exclusive.) None of the elements in the metadata take a "ref" attribute; to provide additional information, use the "description" element instead.
5.4.2. The "comment" Attribute
Any "char", "range", or "variant" element in the "data" section may contain an OPTIONAL "comment" attribute. The contents of a "comment" attribute are free-form plain text. Comments are ignored in machine processing of the table. "comment" attributes MAY also be placed on all elements in the "rules" section of the document, such as actions and match operators, as well as definitions of classes and rules, but not on child elements of the "class" element. Finally, in the metadata, only the "version" and "reference" elements MAY have "comment" attributes (to match the syntax in [RFC3743]).5.5. Code Point Tagging
Typically, LGRs are used to explicitly designate allowable code points, where any label that contains a code point not explicitly listed in the LGR is considered an ineligible label according to the ruleset. For more-complex registry rules, there may be a need to discern one or more subsets of code points. This can be accomplished by applying an OPTIONAL "tag" attribute to "char" or "range" elements that are child elements of the "data" element. By collecting code points that share the same tag value, character classes may be defined (see Section 6.2.2) that can then be used in parameterized context or whole label rules (see Section 6.3.2). Each "tag" attribute MAY contain multiple values separated by white space. A tag value is an identifier that may also include certain punctuation marks, such as a colon. Formally, it MUST correspond to the XML 1.0 Nmtoken (Name token) production (see [XML] Section 2.3). It is an error to duplicate a value within the same "tag" attribute. A "tag" attribute for a "range" element applies to all code points in the range. Because code point sequences are not proper members of a set of code points, a "tag" attribute MUST NOT be present in a "char" element defining a code point sequence.
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